Comment out code for exporting history animation
Ensure that background color is drawn properly when exporting animations
Prevent backspace from triggering the browser's back button

With this kind of app, the user's instinct is to hit backspace when
attempting to delete a selection. If the back button is triggered, that's
basically a catastrophic failure.
Ensure new documents are fully transparent
Ensure that current frame is valid after resizing canvas
Also respond to backspace key when deleting selections. Feels more natural